A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus may be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which may be referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern may be transferred on (part of) the substrate (e.g. a glass plate), e.g. via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
Instead of a circuit pattern, the patterning device may be used to generate other patterns, for example a color filter pattern, or a matrix of dots. Instead of a mask, the patterning device may comprise a patterning array that comprises an array of individually controllable elements. A possible advantage of such a system compared to a mask-based system is that the pattern can be changed more quickly and for less cost. A patterning device may also be used in or as part of an illumination system of a lithographic apparatus, for example for defining an intensity distribution of a radiation beam in a pupil plane of the illumination system.
A patterning device which comprises an array of individually controllable elements is usually driven, or in other words controlled by a driving voltage. For example, if the array of individually controllably elements comprises mirrors, the degree of tilt of each of the mirrors of the array can be controlled by appropriate control of the driving voltage for that mirror. For instance, in order to tilt a mirror of the array by a certain angle, a specific voltage can be applied between the mirror and an electrode adjacent to the mirror. The applied voltage may be a non-varying DC voltage. However, it has been found that the use of a DC driving voltage may cause surface charges to become trapped and build up in the areas in and around the electrode and possibly the mirror itself. This build up of charge has an effect on the tilt of the mirror which induced by the driving voltage. For example, the build up of charge can cause the degree of tilt of the mirror to drift, even though the driving voltage itself does not change.
In order to solve the problems associated with the use of a DC driving voltage, alternating, or in other words AC, driving voltages have been employed. For example, a square wave AC driving voltage may be used, the voltage varying from a first positive voltage to a second negative voltage. By changing the polarity of the voltage used to drive the mirror, charges do not become trapped and do not build up. Therefore, the problems associated with DC driving voltages are overcome. Furthermore, since electrostatic forces are proportional to the square of the applied voltage, the change in the polarity of the AC driving voltage does not affect the force experienced by the mirror, and therefore the degree of tilt of the mirror.
Although the use of an AC driving voltage overcomes at least some of the problems associated with the use of a DC driving voltage, the use of an AC driving voltage still has disadvantages associated with it. For instance, in some arrangements where the array of individually controllable elements is used to form a pattern in the cross-section of the radiation beam, the radiation beam is formed from a plurality (e.g. thousands or millions) of radiation beam pulses. It is possible that a radiation beam pulse may be incident upon an element of the array of individually controllable elements at a time when the driving voltage changes polarity. When the driving voltage changes polarity, there may be a short period of time when the tilt of the mirror changes. A change in the tilt of the mirror will have a consequential effect on the direction of reflection of radiation beam pulse incident upon and then reflected from the mirror. If, as is usually the case, the radiation beam pulse should be directed to a high degree of accuracy, unexpected changes in the tilt of the mirror should be avoided.
In another example, it has been found that elements of the array of individually controllable elements are excited by the radiation beam pulse (i.e. from photon impulse or energy). Frequency mixing between the AC driving voltage and the radiation beam pulse frequency can result in mixed frequency terms due to a non-linear relationship between the position or orientation of the elements and the voltage that is being used to drive them. The mixed frequency terms may excite the (mechanical) eigenmodes of the elements of the array of individually controllable elements.
In yet another example, electronics used to provide or control the AC driving voltage usually have a fixed operating voltage range. In the case of a DC driving voltage, the full operating voltage range may be used to drive the elements of the array of individually controllable elements. However, when an AC driving voltage is used, the maximum swing (i.e. from maximum positive to maximum negative voltage) is equal to the maximum operating voltage range. However, the absolute applied modulation voltage is only half of that of the maximum voltage range. This means that, using the same electronics as the DC driving voltage, the force generated (and therefore the consequential tilt or movement of elements of the array) is only a quarter of that achievable using a DC driving voltage. This is because the absolute maximum driving voltage for an AC driving signal will be half of that of a DC driving signal, and the force generated is proportional to the square of the driving voltage.